Relative rates of appearance of nitrogen and sulphur: implications for postprandial synthesis of proteins

Abstract

The purpose of this study was to gain insights on the temporal fate of proteins based on the rate of appearance of waste products of nitrogen (urea) and sulphur (sulphate) metabolism. Urine was collected every 2 h from 25 normal subjects to measure the rates of excretion of urea, creatinine, and sulphate throughout the 24-h cycle. Samples of blood and urine were also obtained over a 4-h period from 10 subjects who consumed a mixed meal containing 0.4 g protein/kg body weight to obtain information on the relative rates of degradation of amino acids with and without sulphur in a noninvasive fashion. The daily excretion (mean ± SEM) of urea, creatinine, and sulphate was 396 ± 28, 14 ± 0.4, and 15 ± 0.6 mmol, respectively; the molar sulphate/nitrogen (S/N) ratio was 2.0 ± 0.1%. There were relatively minor (<20%) excursions in the rate of excretion of urea and creatinine in any 2-h period as compared with the corresponding 24-h rate; the concentrations of urea and creatinine in plasma also varied <20% throughout the day. Only 23% of the nitrogen in protein in the standard meal appeared as urea in the 210 min after this meal was consumed. The small changes in the rate of appearance of urea and creatinine imply that the oxidation of amino acids was spread out over the day. In contrast to urea and creatinine, the rate of appearance of sulphate underwent a greater variation; in general, there was a nadir just after breakfast and a peak overnight (7.0 ± 0.6 and 14.3 ± 1.6 μmol/min, respectively), and a S/N appearance rate that rose from 1.3 to 2.6%, suggesting specific retention of sulphur-containing compounds during the day, with catabolism occurring many hours later. When there was a low intake of protein for 40 h, the S/N appearance rate changed appreciably: the mean value fell in the last 24 h from 1.8 ± 0.1 to 1.2 ± 0.1% when carbohydrate was consumed, whereas it was 2.3 ± 0.1% during a 40-h fast. We conclude that most of the amino acids of dietary origin are not catabolized directly after a meal in normal subjects and that sulphur in proteins or other compounds is retained for longer periods following meals. During a 40-h fast, net protein catabolism includes proteins rich in of sulphur-containing amino acids, whereas the converse occurs when subjects consumed the low protein – high carbohydrate diet.Key words: amino acids, creatinine, diurnal variation, nitrogen balance, protein turnover, sulphate, urea.